1. Field of the Invention
The present invention relates to improving the oxidation stability of lubricants, especially hydrocarbon based lubricating oils, by adding thereto a combination of at least two antioxidants.
2. Description of Related Art
Lubricants, such as those used in a variety of machinery, are susceptible to oxidative deterioration during storage, transportation, and usage, particularly when such lubricants are exposed to high temperatures and iron catalytic environments, which greatly promote their oxidation. This oxidation, if not controlled, contributes to the formation of corrosive acidic products, sludge, varnishes, resins, and other oil-insoluble products, and may lead to a loss of designated physical and tribological properties of the lubricants. It is therefore a common practice to include an antioxidant in lubricants to prevent, at least to some extent, oxidation, so as to extend their useful life. Lubricant compositions containing various diarylamines or phenolic compounds as antioxidants are widely known in the art.
U.S. Pat. No. 2,718,501 discloses a stabilizer system consisting of an aromatic amine with at least two aromatic rings, including para-phenylenediamine, and an organic aliphatic sulfur compound, which is said to be suitable for stabilizing mineral hydrocarbon lubricating oils, synthetic hydrocarbon oils, and polyalkylene glycol oils.
U.S. Pat. No. 5,091,099 discloses a phosphite-free lubricating oil composition which comprises a mineral oil or a synthetic oil or a mixture thereof, and a mixture containing at least one aromatic amine and at least one phenol. The lubricating oil compositions are said to be highly resistant to aging and are said to be effective in preventing black sludge formation.
U.S. Pat. No. 5,229,442 discloses stabilizing compositions for organic polymers, composed of mixtures of at least one liquid phenolic antioxidant and at least one aminic antioxidant, subjected to thermal treatment, that are characterized by their stabilizing capacity, which is considerably higher than that of either single components or of corresponding untreated mixtures. These stabilizing compositions can be used in all fields where the single components of the mixtures are generally used and, preferably, in the stabilization of organic polymers normally subject to deterioration due to oxidation processes.
U.S. Pat. No. 5,523,007 discloses a lubrication system comprising a diesel engine lubricating oil which is stabilized with an ester of a sterically hindered phenolic compound and the use of an ester of thio-ester compound for stabilizing diesel engine lubricating oils.
WO 94/22988 discloses a fuel composition said to improve the antiwear and viscosity controlling properties of an internal combustion engine lubricating oil during operation of the engine. Small amounts of the fuel composition combine with the engine lubricating oil during engine operation and this provides an antioxidant boost to the lubricating oil. Preferably the fuel contains at least 57 g/1000 liters of a substituted dicyclic aromatic amine which is free of benzylic hydrogen atom s such as a mono- and/or di-α-methyl styrene alkylated phenylenediamine and/or a hindered phenol such as a monostyrenated mono-isobutenated cresol or di C16 alkyl phenol. A synergistic effect is said to be demonstrated by a mixture of the aromatic amine and hindered phenol.
JP 53,051,206 discloses N,N′-2-naphthyl-p-phenylenediamine as an antioxidant to improve the oxidation stabilities of ester or mineral oil based lubricating oils that also contain disulfides.
JP 59,020,392 discloses a lubricant composition comprising N,N′-di-sec-butyl-p-phenylenediamine for pressure forming of oil tanks. The lubricant composition also contains hindered phenolic antioxidant.
The foregoing disclosures are incorporated herein by reference in their entirety.